Tumor promoters can often be differentiated by their effects in two model systems: polynuclear aromatic hydrocarbon (PAH)-initiated carcinogenesis in mouse skin and nitrosamine-initiated carcinogenesis in regenerating rat or mouse liver. Phorbol esters promote in both systems, some phthalate esters promote the latter and fail to affect the former, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) promotes the latter and is an antipromoter for the former, while polychlorinated biphenyls (PCBs) may promote or act as an antipromotor for both depending on the dosages tested. A comparison of these classes of environmental pollutants may provide mechanistic insights absent when any one is studied by itself. This is a new project that is still largely in the planning and preparation stages.